A conventional conductive metal paste utilizing metal powders as a conductive medium has attempted to achieve electrical contact among the metal powders as well as to achieve adhesion to a surface of a substrate by blending an adequate amount of a binder resin component into the metal powders and then by curing this binder resin component. On the other hand, instead of using such metal powders, utilization of extremely fine metal particles having an average particle size of 100 nm or less, that we call metal nanoparticles, allows for mutual sintering of the metal fine particles by subjecting them to a heat treatment at a relatively low temperature. Practical use of characteristics which are inherent to the metal nanoparticles has led to a development of various kinds of conductive metal pastes which are used for fabricating a conductive layer comprising a layer of metal nanoparticle sintered product.
In a field involved in recent electronic equipment, there has progress in miniaturization of a wiring pattern on a wiring substrate to be used. In addition, also in respect to a metal thin layer used for forming various kinds of electrode pattern parts, there has progress in utilization of a metal thin layer having an ultrathin thickness. In this case, it has extensively been studied the utilization of a metal nanoparticle sintered product layer which is formed by using a conductive metal paste containing metal nanoparticles as a conductive medium, instead of utilizing a metal thin layer formed by a plating method. When fine wiring formation or thin layer formation is achieved by using a screen printing process for example, it is attempted to make use of a dispersion of metal fine particles having an extremely small particle size in order to graphically draw an ultrafine pattern or to form a thin film coated layer having an extremely thin thickness. At the present moment, a dispersion of gold or silver urtrafine particles which is applicable to the above described use has already been commercialized.
When silicate glass or the like is used as a substrate material, many kinds of metals do not exhibit a favorable adhesion characteristic for the flat glass surface, therefore, in the case of a metal film layer formed by a plating method for example, a metal thin layer having a high adhesion with respect to the glass material is beforehand applied to the glass surface, and this coating is used as a plating underlying film layer. Since a metal element to be plated is precipitated out on a surface of a metal which constitutes the plating underlying film layer, the plating underlying film layer and a metal film layer formed by a plating method are strongly adhered to each other via bonding between metal atoms which is created at an interface between the plating underlying film layer and the metal film layer formed by a plating method.
On the other hand, although a metal nanoparticle sintered product layer which is formed by using a conductive metal paste is densely applied to a flat glass surface as a whole, a region in which the flat glass surface comes into contact with the individual metal nanoparticles is small in area from a microscopic point of view. Therefore, adhesion characteristic of the entire metal nanoparticle sintered product layer may become insufficient depending on its use, even if a metal species used as the metal nanoparticle has a high adhesion with respect to the glass material. For example, if a thermal expansion coefficient of the glass material used as a substrate is different from a substantial thermal expansion coefficient of the metal nanoparticle sintered product layer to be formed and a heating temperature during a sintering treatment is set high, and then a strain stress is accumulated at an interface after cooling down to a room temperature. The strain stress accumulated at the interface tends to be concentrated in a region in which the flat glass surface comes into contact with the individual metal nanoparticles from a microscopic point of view, and consequently the metal nanoparticle sintered product layer may partly be stripped off the flat glass surface.
In addition, a wiring pattern formed on the substrate surface is subject to changes in ambient temperature during its implementation and its use, and further, the strain stress caused by the temperature changes is repeatedly applied to an interface between the metal nanoparticle sintered product layer and the substrate surface. As a result of repeated application of the strain stress to the interface after experiencing several cycles of ambient temperature changes, a part of the metal nanoparticle sintered product layer may frequently be stripped off the flat glass surface even if there are no situations that the metal nanoparticle sintered product layer is partly stripped off the flat glass surface at the beginning of the fabrication process.
A pre-treatment in which a flat glass surface is uniformly coated with a metal layer having high adhesion with respect to a glass material so as to form an underlying metal film layer in advance allows for avoiding and suppressing a phenomenon of partly stripping off a metal nanoparticle sintered product layer, because a strain stress caused at an interface is distributed throughout the underlying metal film layer and a favorable adhesion is provided between the individual metal nanoparticles and the underlying metal film layer. Further, also suggested is that adhesion between a metal nanoparticle sintered product layer formed by blending an adequate amount of a binder resin component into a conductive metal paste containing metal nanoparticles as a conductive medium and a substrate surface is principally provided via a cured binder resin (International Publication WO02/035554 A1). Use of this method, in which an adequate amount of the binder resin component is blended and then timing of sintering the metal nanoparticles is adjusted to timing of curing the binder resin component, achieves formation of a metal nanoparticle sintered product layer exhibiting an excellent conductivity and provision of a high adhesion characteristic given by the blended binder resin.
According to a method which conducts a pre-treatment of forming an underlying metal film layer for example, the pre-treatment in which a uniform underlying metal film layer is beforehand coated throughout a surface of a substrate is performed in combination with a post-treatment in which an excess portion of the underlying metal film layer is etched away after completing the formation of a metal nanoparticle sintered product layer, prior to lithographically drawing a desired fine wiring pattern with a conductive metal paste containing metal nanoparticles through the use of screen printing for example. Thus, this process as a whole still has complications similar to those in the case of a mask plating method. On the other hand, according to a method in which an adequate amount of a binder resin component is blended and then timing of sintering metal nanoparticles is adjusted to timing of curing the binder resin component, complications during this process may be eliminated, however, it is important to properly combine composition of a conductive metal paste per se with conditions of a heat treatment (temperature, time) in order to adjust timing of sintering the metal nanoparticles with timing of curing the binder resin component. Specifically, this process experiences more difficulty in setting a temperature for the heat treatment to a high degree as well as in adjusting timing of sintering the metal nanoparticles to timing of curing the binder resin component.